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Recent efforts have been concentrated in the development of models to understand and predict the impact of environmental changes on hydrological cycle and water resources in arid and semi-arid regions. In this context, remote sensing data have been widely used to initialize, to force, or to control the simulations of these models. However, for several reasons, including the difficulty in establishing relationships between observational and model variables, the potential offered by satellite data has not been fully used. As a matter of fact, a few hydrological studies that use remote sensing data emanating from different sources (sensors, platforms) have been performed. In this context, the SUDMED programme has been designed in 2002 to address the issue of improving our understanding about the hydrological functioning of the Tensift basin, which is a semi-arid basin situated in central Morocco. The first goal is model development and/or refinement, for investigating the hydrological responses to future scenario about climate change and human pressure. The second aim is the effective use of remote sensing observations in conjunction with process models, to provide operational prognostics for improving water-resource management. The objective of this paper is to present the SUDMED programme, its objectives, and its thrust areas, and to provide an overview of the results obtained in the first phase of the programme (2002-2006). Finally, the lessons learned, future objectives, and unsolved issues are presented. [ABSTRACT FROM AUTHOR]

Evapotranspiration plays a crucial role in the surface hydrothermal balance. On a regional scale, especially in the blown-sand areas, an accurate estimate of evapotranspiration has important guiding significance for a rational allocation of water resources and drought monitoring. The objective of the study is to estimate the surface evapotranspiration for the blown-sand region in the Ordos Basin based on the Surface Energy Balance Algorithm for Land (SEBAL) model using six phases of Landsat 8 remote sensing images during 2015-2016 and the corresponding surface meteorological observational data, analyze the characteristics of temporal and spatial distributions of evapotranspiration in the study area. The results show that (1) the daily evapotranspiration presents a seasonal variability with the highest in summer, lower in autumn and spring, and the lowest in winter; (2) the evapotranspiration is distinctly characterized by its spatial variability, which is related to the terrain and geomorphology in this area as well as the effect of human activities; and (3) the daily evapotranspiration estimated by the SEBAL model was validated using the Food and Agriculture Organization (FAO) Penman-Monteith (P-M) model, which yielded an mean relative error (MRE) of 19.5%. In summary, the remote sensing data based SEBAL model can be applied to robustly estimate evapotranspiration for the blown-sand regions, and the study results can be served as an important reference for the research of water circulation, rational utilization of water resources, and protection of the ecological environment in the arid and semi-arid regions. [ABSTRACT FROM AUTHOR]

Due to the increase in urban and agricultural activities in arid regions, the exploration of new locations of possible groundwater discharge and accumulation is required to augment the limited water resources. In order to locate such discharge areas, it is necessary first to identify zones of high recharge potentials. In such an arid region, like the northern United Arab Emirates (UAE), one of the ways to predict areas of potential groundwater recharge is by understanding the hydrological response of its drainage basins to rainfall events. Due to the scarcity of basic hydrological data, a hydrological model driven mainly by information on the physiographic characteristics, drainage network properties (generated from DEM), and surface cover distribution (generated from satellite images) was used to comprehend the dynamics of surface runoff through hydrographs, and hence water loss in the study area. Results show that the northern UAE is drained by 48 drainage basins emerging from the Oman Mountains. Two-thirds of these basins drain easterly toward the Gulf of Oman, and one-third drain westerly toward the Arabian Gulf. These basins are found to be structurally controlled by three major fault trends, which are the NE trend (Dibba zone), NW trend (Ham Zone), and WNW trend (Hatta zone). The hydrological response of a basin is correlated with its morphological characteristics. Based on these characteristics, and through the application of a cluster analysis, it was feasible to classify the largest basins in the region into four groundwater potentiality groups in accordance with the magnitude of their peak discharges. From this study, it became evident that the downstream area of the two major basins of Ar-Rafiah and Limhah, and their vicinities are the most probable sites for groundwater accumulation. The drainage systems of these two basins, especially those controlled by major fault lines, play a vital role in transmitting surface-subsurface rainwater from the Oman Mountains, the recharge zone, into the western desert plain, the discharge zone, where freshwater accumulates underground. The study also revealed that a large volume of groundwater is dissipated into the sea along the eastern coast. A detailed examination of MODIS thermal data supports this by revealing cool surface anomalies issuing from the mountain range toward both the western desert plain and the Gulf of Oman following major rainfall events. Thus, the technique used facilitates the prediction of new locations of optimum groundwater resources in the northern UAE. Such a technique could be adopted, with appropriate modifications, elsewhere in arid regions, where groundwater is restricted and subject to greater complexity. [ABSTRACT FROM AUTHOR]

In the framework of the WATERMED (WATer use efficiency in natural vegetation and agricultural areas by Remote sensing in the MEDiterranean basin) project, an experimental field campaign was carried out in a semi-arid region near Marrakech, Morocco, during March 2003. This work focused on the columnar aerosol characterization from spectroradiometric and photometric measurements of direct solar irradiance and sky radiance at ground level. The results show a high dependence of the aerosol optical properties on the air masses present in the area. Two periods with different aerosol loads were observed in the campaign. In the first, Atlantic-origin aerosols showed a marine particle accumulation mode combined with a dominant mode corresponding to coarse particles of local origin. In the second period the synoptic situation changed towards a Saharan+Atlantic origin, the Atlantic influence persisting at altitude while at lower levels the presence of air masses proceeding from the Saharan desert was observed. This dry air carried a large quantity of mineral particles that increased the area's turbidity, leading to a substantial increase in the volume of the whole distribution, a displacement of the accumulation mode radius towards higher values and an increasingly evident spread of the coarse particle mode, although the radius remained the same as that in the first period of the campaign. [ABSTRACT FROM AUTHOR]